What Went Wrong in Evaluating the Limit Using L'Hospital's Rule?

  • Thread starter SpockTock
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In summary, the conversation discussed the problem of finding the limit as x approaches infinity of xln((x+3)/(x)), which is an indeterminate form. It was solved using a logarithmic manipulation and the power series for ln. The solution came out to be negative infinity, but the graph showed a limit of three. The person asked for help in identifying their mistake.
  • #1
SpockTock
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I came across the problem lim as x approaches infinity of xln((x+3)/(x)). This is an indeterminate limit so L'Hospitals rule must be used to evaluate it. I solved it to come out to negative infinity, but the graph says it's three. What did I do wrong in solving it?
 
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  • #2
Can I see what you've done?
 
  • #3
write
x log((x+3)/x)=3 [log(1+3/x)-log(1)]/(3/x)
then you won't need L'Hospital's Rule which would work despite being unneeded. As far as what the mistake was how would anyone know?
 
  • #4
You can also use the power series for ln: ln(1 + 3/x) = 3/x +O(1/x2)
 

FAQ: What Went Wrong in Evaluating the Limit Using L'Hospital's Rule?

What is L'Hospital's Rule for products?

L'Hospital's Rule for products is a mathematical rule used to evaluate limits of indeterminate products, where both the numerator and denominator approach zero or infinity. It states that the limit of a product is equal to the product of the limits of the individual factors.

What is an indeterminate product?

An indeterminate product is a mathematical expression in which the product of two variables or functions approaches a value that cannot be determined by simply plugging in the values of the variables or evaluating the functions at a certain point.

When is L'Hospital's Rule for products used?

L'Hospital's Rule for products is used when evaluating limits of indeterminate products, specifically when both the numerator and denominator approach zero or infinity. It is also used when the limit of a function can be rewritten as an indeterminate product.

What are the conditions for applying L'Hospital's Rule for products?

The conditions for applying L'Hospital's Rule for products are:

  • The limit is indeterminate, with both the numerator and denominator approaching zero or infinity.
  • The limit can be rewritten as an indeterminate product.
  • The numerator and denominator are both differentiable functions.
  • The derivative of the numerator and denominator both exist at the point of evaluation.
  • The limit of the derivative of the numerator and denominator exists or is infinite at the point of evaluation.

Can L'Hospital's Rule for products be applied multiple times?

Yes, L'Hospital's Rule for products can be applied multiple times, as long as the conditions for applying the rule are still met after each application. This is known as repeated application of L'Hospital's Rule.

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